Such pumps already exist which, in a standard manner, comprise a first cylinder forming a pump chamber, which communicates with an intake tube by means of a check valve, and in which a hollow piston is slidingly mounted. They also comprise a second cylinder, which communicates with the first cylinder, and in which a second piston formed by a needle valve is slidingly mounted, capable of interrupting the communication between the first cylinder and a dispensing nozzle. They finally comprise elastic means, such as a compression spring, which act on the needle valve tending to maintain the latter in a blocking position in which said communication is interrupted.
Such pumps have the following operating principle: when the first piston is moved, the liquid located in the first cylinder is forced into the second cylinder, in which the pressure increases. The second piston tends to push into its cylinder, compressing the spring. When the pressure of the liquid reaches a high enough value to balance this action, the second piston moves, driving the valve linked to it, so that the cylinders are placed in communication with the dispensing nozzle.
Such pumps do not comply with the present demands of quality standards since, for example, the liquid product can be polluted through contact with the metal spring that returns the needle valve to blocking position after dispensing the liquid product.
Furthermore, the doses of liquid product dispensed are often inaccurate, which limits the use of these pumps to the dispensing of certain products.
Moreover, the designs of these pumps do not allow the pump to be produced in several models capable of dispensing the product with different dose sizes. In fact, the volume of the chamber cannot be changed without changing the dimensions of the pump. Thus, it is necessary to provide specific packaging for each size of pump, which considerably increases the production costs.